ACE2 could mediate  SARS-CoV-2 S-mediated entry into cells, establishing it as a functional receptor for this newly emerged coronavirus. The SARS-CoV-2 SB engages human ACE2 (hACE2) with comparable affinity to SARS-CoV SB from viral isolates associated with the 2002-2003 epidemic (i.e., binding with high affinity to hACE2). Tight binding to hACE2  could partially explain the efficient transmission of SARS-CoV-2 in humans, as was the case for SARS-CoV.